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Efficacy of doramectin against field nematode infections of cattle in Latin America
Veterinary Parasitology, 49 ( 1 9 9 3 ) 3 9 - 4 4 0 3 0 4 - 4 0 1 7 / 9 3 / $ 0 6 . 0 0 © 1993 - Elsevier Science Publishers B.V. All rights reserved
39
Efficacy of doramectin against field nematode infections of cattle in Latin America C. Eddi", I. Bianchinb, M.R. Honerb, R.A. Muniz*'c, J. Caracostantogolo a, Y.A. d o N a s c i m e n t o b alnstituto de Patobiologia CICV-INTA, CC77-1708 Moron, Argentina bEMBRAPA/CNPGC, Campo Grande, MS, Brazil CAnirnal Health Group, Pfizer Inc., Eastern Point Road, Groton, CT 06340, USA
Abstract Two studies were conducted, one in Argentina and one in Brazil, to evaluate the therapeutic efficacy of doramectin administered subcutaneously at a dose rate of 200 #g kg-1 (1 ml per 50 kg) to cattle harboring mixed field infections of gastrointestinal nematodes. In each trial, 20 heifers were randomly allocated to a control or treated group of ten animals each, on the basis of fecal egg counts. Animals in the treated and control groups received an injection of doramectin or saline in the lateral midline of the neck. From 14 to 18 days post-treatment an equal number of animals from each group were slaughtered every day and the worm burdens of doramectin-treated animals were compared with those of non-treated controls. The efficacy of doramectin was at least 99.9% against adult stages of Osterta-
gia ostertagi, Haemonchus placei, Haemonchus contortus, Haemonchus sirnilis, Trichostrongylus axei, Trichostrongylus longispicularis, Cooperia oncophora, Cooperia punctata, Cooperia pectinata, Cooperia spatulata, Cooperia surnabada (Cooperia mcrnasteri), Oesophagostomum radiaturn and Dictyocaulus viviparus. Efficacy against Nematodirus helvetianus was 97.9% while efficacy against Trichuris discolor was 92.3%. Activity against inhibited larval forms of Ostertagia ostertagi, H. placeL Oesophagostomum radiatum, and Trichostrongylus axei, was at least 99.9%.
Introduction Economic losses due to internal parasites are extensive in the cattle populations of Brazil and Argentina. Entrocasso (1988) estimated losses at $ 22 million in grazing cattle from trichostrongyle infections in Argentina. Bianchin et at. (1990) indicated that helminth parasites were one of the principal causes of poor performance in the 132 million cattle of Brazil. The anthelmintic efficacy of doramectin (Dectomax TM,Pfizer) has been well established for a broad range of gastrointestinal nematodes and lungworms (Jones et al., 1993 ) with controlled studies in large numbers of cattle. These studies were conducted in the Northern Hemisphere under both laboratory and field conditions using species of economic importance in the re*Corresponding author.
40
c. Eddi et al. / Veterinary Parasitology 49 (1993) 39-44
gion. Previous publications have described both species and population distribution differences between nematode parasites of cattle in the Northern Hemisphere and those of the Southern Hemisphere (Guerrero and Leaning, 1990). This paper describes studies designed to evaluate the efficacy of doramectin against nematode parasites acquired under conditions of natural exposure by grazing cattle in Argentina and Brazil. Materials and methods
Study sites and animals Studies were conducted during the summer in Argentina and during the spring in Brazil. The trial in Argentina was carried out on the premises of the Veterinary Sciences Investigations Center (CICV), INTA-Castelar, while the Brazil trial was conducted in the National Center of Beef Cattle Research at EMBRAPA/CNPGC, Campo Grande. In each trial, 20 heifers (Nellore breed in Brazil, Brangus breed in Argentina) harboring field gastrointestinal nematode infections were selected from a herd in which the presence of infection in the herd was confirmed by postmortem examination of five animals. In these sentinel animals, adult worms were the predominant stage for all species except for Haemonchus placei, Ostertagia ostertagi, Oesophagostomum radiatum and Trichostrongylus axei where immature fourth-stage larvae (L4) (or third-stage larvae (L3) for T. axei) were present.
Experimental design After a 7 day period of acclimatization to individual pens and dry grass hay, heifers were weighed and ranked on the basis of fecal egg counts. Animals in ranked pairs were randomly allocated to each of two groups (T1, nonmedicated control; T2, doramectin treated group) of ten animals each (Roberts and O'Sullivan, 1949 ). On the day of treatment (Day 0), each heifer in the treated group was injected with doramectin at 200/zg kg -~ body weight ( 1 ml per 50 kg) subcutaneously in the lateral midline of the neck and controls were injected with saline at 1 ml per 50 kg. From 14 to 18 days after treatment, equal numbers of animals from each group were randomly selected and slaughtered and their lungs, abomasum, small and large intestines were processed for worm recovery.
Parasitological techniques Necropsy of cattle, collection of worms from specific gastrointestinal compartments including abomasal saline incubation at 37 °C, aliquoting of sam-
C. Eddi et al. / Veterinary Parasitology 49 (1993) 39-44
4[
pies and identification of species were all done by standard parasitological techniques as described by Powers et al. (1982). A m i n i m u m aliquot volume of 10% was counted. The analysis of lung tissue was performed as described by Oakley (1980). Fourth stage larvae of Ostertagia, Haemonchus and Oesophagostomum that were found after 21 days removal from pasture were assumed to be inhibited.
Statistical analyses Percentage efficacy for each species of nematode (PE) was calculated using the following formula (Powers et al., 1982) Geometric mean worm counts in control cattle Geometric mean worm counts in treated cattle PEX 100 Geometric mean in control cattle In the control group, animals with no specific parasite burden were excluded from the analysis. An equal proportion of animals with zero burden of the same parasite was also excluded from the doramectin-treated group. Statistical analyses were carried out using a linear model to partition the total variation in log worm burdens into sources that were functions of the design and treatment structure of the experiment. The error term (i.e. the between-animal within-treatment group variation) was then used to test the statistical significance ( P < 0.05) of the difference between treatments in their mean log worm burdens. Results Table 1 reports the geometric mean of adult worm recovery from the gastrointestinal tract of doramectin-treated and control cattle from a trial conducted in Argentina. Percentage efficacy is calculated for group mean reductions in worm burdens. A highly significant ( P < 0 . 0 0 0 1 ) reduction of
Ostertagia ostertagi, Trichostrongylus axei, H. placei, Cooperia oncophora, Cooperia surnabada (syn. mcmasteri ), Cooperia punctata, and Oesophagostomum radiatum were observed in the treated group. The efficacy of doramectin against all these parasites was at least 99.9% (Table 1 ). Efficacy against
Nematodirus helvetianus was 97.9%. In addition to the above species, small numbers of adult stages of Ostertagia lyrata and Trichostrongylus colubriformis were also recovered in the control group but not in the treated group; however, the incidence and level of infection was too low to allow the estimation of efficacy. The efficacy of doramectin against the c o m m o n adult worms of cattle found
c: Eddi et aL / VeterinaryParasitology 49 (1993) 39-44
42
Table 1 Geometric mean adult worm burdens and percentage efficacy (PE) following doramectin treatment ( 200/~g kg- 1) of cattle in Argentina Parasite
n~
Geometric mean worm burden
PE
P value 2
Ostertagia ostertagi Trichostrongylus axei Haemonch us placei Cooperia oncophora Cooperia surnabada Cooperiapunctata Nematodirus helvetianus Oesophagostomum radiatum
10 I0
Control group
Doramectin group
4301 3092
1 0 0 3 0 0 9 0
10
410
10 8 10 8 8
4325 400 782 430 30
99.9 100.0 100.0 99.9 100.0 100.0 97.9 100.0
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
~Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
Table 2 Geometric mean and percentage efficacy (PE) of adult worms recovered following doramectin treatment (200/tg kg- t ) of cattle in Brazil Parasite
n~
Geometric mean worm burden
PE
P value 2
Trichostrongylus axei Trichostrongylus longispicularis Cooperia pectinata Cooperiapunctata Cooperia spatulata Cooperia spp. Haemonchus contortus Haemonchus similis Haemonchus spp. Oesophagostomum radiatum Trich uris discolor Dictyocaulus viviparus
10 9
Control group
Doramectin group
3290 178
1 0 0 0 0 0 0 0 0 0 1 0
10
319
10 4 10 10 10 4 10
3686 230 6331 248 120 22 782
10
13
10
226
99.9 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 92.3 100.0
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
~Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
in Brazil, is presented in Table 2. A highly significant ( P < 0.0001 ) reduction of at least 99% of adult stages of Cooperia pectinata, C. punctata, Cooperia spatulata, Cooperia spp., Dictyocaulus viviparus, Haemonchus contortus, Haemonchus similis, Haemonchus spp., Oesophagostomum radiatum, Trichostrongylus axei, and Trichostrongylus longispicularis, was observed in the
C Eddi et al. / VeterinaryParasitology 49 (1993) 39-44
43
Table 3 Geometric mean and percentage efficacy (PE) of inhibited worms recovered following doramectin treatment (200/zg kg -~ ) of cattle in Argentina (A) and Brazil (B) Parasite
Location
~/i Geometric mean worm burden Control group
Ostertagia ostertagi Trichostrongylus axei Haemonchus placei Cooperia oncophora Trichostrongylus axei Oesophagostomum radiatum
A A A A B B
10 3995 7 370 8 74 7 87 8 44 10 184
PE
P value 2
99.9 100.0 100.0 100.0 100.0 100.0
<0.0001 < 0.0001 < 0.0001 <0.0001 < 0.0001 <0.0001
Doramectin group 0.4 0 0 0 0 0
1Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
cattle treated with doramectin when compared with the non-medicated control group. Efficacy against Trichuris discolor was 92.3%. In addition to the above species, small numbers of adult stages of Bunostomum phlebotomum were also observed in control animals. Although no B. phlebotomum were observed in treated cattle, the incidence of infection was too low to calculate efficacy. Geometric means of L4 stages recovered from the gastrointestinal tract and percentage efficacy after treatment with doramectin in the trials carded out in Argentina and Brazil, are shown in Table 3. Doramectin was at least 99.9% effective in the removal of inhibited stages of Ostertagia ostertagi, H. placei, C. oncophora, Oesophagostomum radiatum and Trichostrongylus axei. The number of L4 stages recovered in the treated group was significantly lower ( P < 0.0001 ) than in the non-medicated control group. Discussion In the present studies a single subcutaneous injection of doramectin administered at a dose rate of 200 #g kg- 1body weight was highly efficacious against natural infections of adult stages of Ostertagia ostertagi, Trichostrongylus axei,
Trichostrongylus longispicularis, H. contortus, H. similis, H. placei, C. oncophora, C. punctata, C. pectinata, C. spatulata, C. surnabada, D. viviparus, N. helvetianus, Oesophagostomum radiatum, and Trichuris discolor. Doramectin was also shown to be highly effective against inhibited larval stages of Ostertagia ostertagi, H. placei, C. oncophora, Oesophagostomum radiatum and Trichostrongylus axei. The high levels of efficacy against H. similis and H. contortus (100%) and Trichuris discolor (92%) supplement the extensive list of parasitic nematodes
44
c. Eddi et aL / Veterinary Parasitology 49 (1993) 39-44
against which doramectin has previously been demonstrated to be active (Jones et al., 1993 ). Efficacy against N. helvetianus, the dose-limiting species for ivermectin (Benz et al., 1989), approached 98%. This 98% efficacy against natural infections o f N. helvetianus m a y indicate that efficacy against field populations is more complete than for laboratory isolates (Jones et al., 1993 ). Drug action and i m m u n e response from a natural field infection m a y contribute to greater efficacy than is found with single dose induced infection in the laboratory. The n e m a t o d e species r e m o v e d from doramectin-treated cattle following injection o f 2 0 0 / t g k g - 1 confirm its therapeutic efficacy to remove natural n e m a t o d e infections acquired u n d e r tropical and subtropical field grazing conditions. Species o f economic importance shared by the Northern and Southern Hemispheres, as well as species more c o m m o n to the Southern Hemisphere, were effectively removed. The demonstrated endoparasitic activity o f doramectin should add to the control o f economic losses due to gastrointestinal nematodes and lungworms infecting cattle in the Southern Hemisphere.
References Benz, G.W., Roncalli, R.A., and Gross, S.J., 1989. Use ofivermectin in cattle, sheep, goats, and swine. In: W.C. Campbell (Editor), Ivermectin and Abamectin. Springer, New York, 360 PP. Bianchin, I., Honer, M.R. and de Nascimento, Y.A., 1990. The epidemiologyof helminths in Nellore beef cattle in the Cerrados of Brazil. In: J. Guerrero and W.H.D. Leaning (Editors), Proc. Syrup. MSD Ag Vet, Epidemiologyof Bovine Nematode Parasites in the Americas, August 1987, Montreal, Canada. Veterinary Learning Systems, 118 pp. Entrocasso, C., 1988. Epidemiologyand control of bovine ostertagiasis in South America. Vet. Parasitol., 27: 59-65. Guerrero, J. and Leaning,W.H.D. (Editors), 1990. Proc. Symp. MSD Ag Vet, Epidemiologyof Bovine Nematode Parasites in the Americas, August 1987, Montreal, Canada. Veterinary Learning Systems, 118 pp. Jones, R.M., Logan, N.B., Weatherley,A.J., Little, A.S. and Smothers, C.D., 1993. Activity of doramectin against nematode endoparasites of cattle. Vet. Parasitol., 49: 27-37. Oakley, G.A., 1980. The recovery of Dictyocaulus viviparus from bovine lungs by lung perfusion: a modification of Inderbitzen's method. Res. Vet. Sci., 29: 395-396. Powers, K., Wood, L., Eckert, J., Gibson, T. and Smith, H., 1982. World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) guidelines for evaluating the efficacy ofanthelmintics in ruminants (bovine and ovine). Vet. Parasitol., 10: 205-264. Roberts, F. and O'Sullivan, P., 1949. Methods for egg counts and larvae cultures for strongyles infesting gastrointestinal tract of cattle. Aust. J. Agric. Res., 1: 99-102.
39
Efficacy of doramectin against field nematode infections of cattle in Latin America C. Eddi", I. Bianchinb, M.R. Honerb, R.A. Muniz*'c, J. Caracostantogolo a, Y.A. d o N a s c i m e n t o b alnstituto de Patobiologia CICV-INTA, CC77-1708 Moron, Argentina bEMBRAPA/CNPGC, Campo Grande, MS, Brazil CAnirnal Health Group, Pfizer Inc., Eastern Point Road, Groton, CT 06340, USA
Abstract Two studies were conducted, one in Argentina and one in Brazil, to evaluate the therapeutic efficacy of doramectin administered subcutaneously at a dose rate of 200 #g kg-1 (1 ml per 50 kg) to cattle harboring mixed field infections of gastrointestinal nematodes. In each trial, 20 heifers were randomly allocated to a control or treated group of ten animals each, on the basis of fecal egg counts. Animals in the treated and control groups received an injection of doramectin or saline in the lateral midline of the neck. From 14 to 18 days post-treatment an equal number of animals from each group were slaughtered every day and the worm burdens of doramectin-treated animals were compared with those of non-treated controls. The efficacy of doramectin was at least 99.9% against adult stages of Osterta-
gia ostertagi, Haemonchus placei, Haemonchus contortus, Haemonchus sirnilis, Trichostrongylus axei, Trichostrongylus longispicularis, Cooperia oncophora, Cooperia punctata, Cooperia pectinata, Cooperia spatulata, Cooperia surnabada (Cooperia mcrnasteri), Oesophagostomum radiaturn and Dictyocaulus viviparus. Efficacy against Nematodirus helvetianus was 97.9% while efficacy against Trichuris discolor was 92.3%. Activity against inhibited larval forms of Ostertagia ostertagi, H. placeL Oesophagostomum radiatum, and Trichostrongylus axei, was at least 99.9%.
Introduction Economic losses due to internal parasites are extensive in the cattle populations of Brazil and Argentina. Entrocasso (1988) estimated losses at $ 22 million in grazing cattle from trichostrongyle infections in Argentina. Bianchin et at. (1990) indicated that helminth parasites were one of the principal causes of poor performance in the 132 million cattle of Brazil. The anthelmintic efficacy of doramectin (Dectomax TM,Pfizer) has been well established for a broad range of gastrointestinal nematodes and lungworms (Jones et al., 1993 ) with controlled studies in large numbers of cattle. These studies were conducted in the Northern Hemisphere under both laboratory and field conditions using species of economic importance in the re*Corresponding author.
40
c. Eddi et al. / Veterinary Parasitology 49 (1993) 39-44
gion. Previous publications have described both species and population distribution differences between nematode parasites of cattle in the Northern Hemisphere and those of the Southern Hemisphere (Guerrero and Leaning, 1990). This paper describes studies designed to evaluate the efficacy of doramectin against nematode parasites acquired under conditions of natural exposure by grazing cattle in Argentina and Brazil. Materials and methods
Study sites and animals Studies were conducted during the summer in Argentina and during the spring in Brazil. The trial in Argentina was carried out on the premises of the Veterinary Sciences Investigations Center (CICV), INTA-Castelar, while the Brazil trial was conducted in the National Center of Beef Cattle Research at EMBRAPA/CNPGC, Campo Grande. In each trial, 20 heifers (Nellore breed in Brazil, Brangus breed in Argentina) harboring field gastrointestinal nematode infections were selected from a herd in which the presence of infection in the herd was confirmed by postmortem examination of five animals. In these sentinel animals, adult worms were the predominant stage for all species except for Haemonchus placei, Ostertagia ostertagi, Oesophagostomum radiatum and Trichostrongylus axei where immature fourth-stage larvae (L4) (or third-stage larvae (L3) for T. axei) were present.
Experimental design After a 7 day period of acclimatization to individual pens and dry grass hay, heifers were weighed and ranked on the basis of fecal egg counts. Animals in ranked pairs were randomly allocated to each of two groups (T1, nonmedicated control; T2, doramectin treated group) of ten animals each (Roberts and O'Sullivan, 1949 ). On the day of treatment (Day 0), each heifer in the treated group was injected with doramectin at 200/zg kg -~ body weight ( 1 ml per 50 kg) subcutaneously in the lateral midline of the neck and controls were injected with saline at 1 ml per 50 kg. From 14 to 18 days after treatment, equal numbers of animals from each group were randomly selected and slaughtered and their lungs, abomasum, small and large intestines were processed for worm recovery.
Parasitological techniques Necropsy of cattle, collection of worms from specific gastrointestinal compartments including abomasal saline incubation at 37 °C, aliquoting of sam-
C. Eddi et al. / Veterinary Parasitology 49 (1993) 39-44
4[
pies and identification of species were all done by standard parasitological techniques as described by Powers et al. (1982). A m i n i m u m aliquot volume of 10% was counted. The analysis of lung tissue was performed as described by Oakley (1980). Fourth stage larvae of Ostertagia, Haemonchus and Oesophagostomum that were found after 21 days removal from pasture were assumed to be inhibited.
Statistical analyses Percentage efficacy for each species of nematode (PE) was calculated using the following formula (Powers et al., 1982) Geometric mean worm counts in control cattle Geometric mean worm counts in treated cattle PEX 100 Geometric mean in control cattle In the control group, animals with no specific parasite burden were excluded from the analysis. An equal proportion of animals with zero burden of the same parasite was also excluded from the doramectin-treated group. Statistical analyses were carried out using a linear model to partition the total variation in log worm burdens into sources that were functions of the design and treatment structure of the experiment. The error term (i.e. the between-animal within-treatment group variation) was then used to test the statistical significance ( P < 0.05) of the difference between treatments in their mean log worm burdens. Results Table 1 reports the geometric mean of adult worm recovery from the gastrointestinal tract of doramectin-treated and control cattle from a trial conducted in Argentina. Percentage efficacy is calculated for group mean reductions in worm burdens. A highly significant ( P < 0 . 0 0 0 1 ) reduction of
Ostertagia ostertagi, Trichostrongylus axei, H. placei, Cooperia oncophora, Cooperia surnabada (syn. mcmasteri ), Cooperia punctata, and Oesophagostomum radiatum were observed in the treated group. The efficacy of doramectin against all these parasites was at least 99.9% (Table 1 ). Efficacy against
Nematodirus helvetianus was 97.9%. In addition to the above species, small numbers of adult stages of Ostertagia lyrata and Trichostrongylus colubriformis were also recovered in the control group but not in the treated group; however, the incidence and level of infection was too low to allow the estimation of efficacy. The efficacy of doramectin against the c o m m o n adult worms of cattle found
c: Eddi et aL / VeterinaryParasitology 49 (1993) 39-44
42
Table 1 Geometric mean adult worm burdens and percentage efficacy (PE) following doramectin treatment ( 200/~g kg- 1) of cattle in Argentina Parasite
n~
Geometric mean worm burden
PE
P value 2
Ostertagia ostertagi Trichostrongylus axei Haemonch us placei Cooperia oncophora Cooperia surnabada Cooperiapunctata Nematodirus helvetianus Oesophagostomum radiatum
10 I0
Control group
Doramectin group
4301 3092
1 0 0 3 0 0 9 0
10
410
10 8 10 8 8
4325 400 782 430 30
99.9 100.0 100.0 99.9 100.0 100.0 97.9 100.0
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
~Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
Table 2 Geometric mean and percentage efficacy (PE) of adult worms recovered following doramectin treatment (200/tg kg- t ) of cattle in Brazil Parasite
n~
Geometric mean worm burden
PE
P value 2
Trichostrongylus axei Trichostrongylus longispicularis Cooperia pectinata Cooperiapunctata Cooperia spatulata Cooperia spp. Haemonchus contortus Haemonchus similis Haemonchus spp. Oesophagostomum radiatum Trich uris discolor Dictyocaulus viviparus
10 9
Control group
Doramectin group
3290 178
1 0 0 0 0 0 0 0 0 0 1 0
10
319
10 4 10 10 10 4 10
3686 230 6331 248 120 22 782
10
13
10
226
99.9 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 92.3 100.0
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
~Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
in Brazil, is presented in Table 2. A highly significant ( P < 0.0001 ) reduction of at least 99% of adult stages of Cooperia pectinata, C. punctata, Cooperia spatulata, Cooperia spp., Dictyocaulus viviparus, Haemonchus contortus, Haemonchus similis, Haemonchus spp., Oesophagostomum radiatum, Trichostrongylus axei, and Trichostrongylus longispicularis, was observed in the
C Eddi et al. / VeterinaryParasitology 49 (1993) 39-44
43
Table 3 Geometric mean and percentage efficacy (PE) of inhibited worms recovered following doramectin treatment (200/zg kg -~ ) of cattle in Argentina (A) and Brazil (B) Parasite
Location
~/i Geometric mean worm burden Control group
Ostertagia ostertagi Trichostrongylus axei Haemonchus placei Cooperia oncophora Trichostrongylus axei Oesophagostomum radiatum
A A A A B B
10 3995 7 370 8 74 7 87 8 44 10 184
PE
P value 2
99.9 100.0 100.0 100.0 100.0 100.0
<0.0001 < 0.0001 < 0.0001 <0.0001 < 0.0001 <0.0001
Doramectin group 0.4 0 0 0 0 0
1Number of control animals harboring infections with the given species. 2Significance level of testing the null hypothesis (Ho).
cattle treated with doramectin when compared with the non-medicated control group. Efficacy against Trichuris discolor was 92.3%. In addition to the above species, small numbers of adult stages of Bunostomum phlebotomum were also observed in control animals. Although no B. phlebotomum were observed in treated cattle, the incidence of infection was too low to calculate efficacy. Geometric means of L4 stages recovered from the gastrointestinal tract and percentage efficacy after treatment with doramectin in the trials carded out in Argentina and Brazil, are shown in Table 3. Doramectin was at least 99.9% effective in the removal of inhibited stages of Ostertagia ostertagi, H. placei, C. oncophora, Oesophagostomum radiatum and Trichostrongylus axei. The number of L4 stages recovered in the treated group was significantly lower ( P < 0.0001 ) than in the non-medicated control group. Discussion In the present studies a single subcutaneous injection of doramectin administered at a dose rate of 200 #g kg- 1body weight was highly efficacious against natural infections of adult stages of Ostertagia ostertagi, Trichostrongylus axei,
Trichostrongylus longispicularis, H. contortus, H. similis, H. placei, C. oncophora, C. punctata, C. pectinata, C. spatulata, C. surnabada, D. viviparus, N. helvetianus, Oesophagostomum radiatum, and Trichuris discolor. Doramectin was also shown to be highly effective against inhibited larval stages of Ostertagia ostertagi, H. placei, C. oncophora, Oesophagostomum radiatum and Trichostrongylus axei. The high levels of efficacy against H. similis and H. contortus (100%) and Trichuris discolor (92%) supplement the extensive list of parasitic nematodes
44
c. Eddi et aL / Veterinary Parasitology 49 (1993) 39-44
against which doramectin has previously been demonstrated to be active (Jones et al., 1993 ). Efficacy against N. helvetianus, the dose-limiting species for ivermectin (Benz et al., 1989), approached 98%. This 98% efficacy against natural infections o f N. helvetianus m a y indicate that efficacy against field populations is more complete than for laboratory isolates (Jones et al., 1993 ). Drug action and i m m u n e response from a natural field infection m a y contribute to greater efficacy than is found with single dose induced infection in the laboratory. The n e m a t o d e species r e m o v e d from doramectin-treated cattle following injection o f 2 0 0 / t g k g - 1 confirm its therapeutic efficacy to remove natural n e m a t o d e infections acquired u n d e r tropical and subtropical field grazing conditions. Species o f economic importance shared by the Northern and Southern Hemispheres, as well as species more c o m m o n to the Southern Hemisphere, were effectively removed. The demonstrated endoparasitic activity o f doramectin should add to the control o f economic losses due to gastrointestinal nematodes and lungworms infecting cattle in the Southern Hemisphere.
References Benz, G.W., Roncalli, R.A., and Gross, S.J., 1989. Use ofivermectin in cattle, sheep, goats, and swine. In: W.C. Campbell (Editor), Ivermectin and Abamectin. Springer, New York, 360 PP. Bianchin, I., Honer, M.R. and de Nascimento, Y.A., 1990. The epidemiologyof helminths in Nellore beef cattle in the Cerrados of Brazil. In: J. Guerrero and W.H.D. Leaning (Editors), Proc. Syrup. MSD Ag Vet, Epidemiologyof Bovine Nematode Parasites in the Americas, August 1987, Montreal, Canada. Veterinary Learning Systems, 118 pp. Entrocasso, C., 1988. Epidemiologyand control of bovine ostertagiasis in South America. Vet. Parasitol., 27: 59-65. Guerrero, J. and Leaning,W.H.D. (Editors), 1990. Proc. Symp. MSD Ag Vet, Epidemiologyof Bovine Nematode Parasites in the Americas, August 1987, Montreal, Canada. Veterinary Learning Systems, 118 pp. Jones, R.M., Logan, N.B., Weatherley,A.J., Little, A.S. and Smothers, C.D., 1993. Activity of doramectin against nematode endoparasites of cattle. Vet. Parasitol., 49: 27-37. Oakley, G.A., 1980. The recovery of Dictyocaulus viviparus from bovine lungs by lung perfusion: a modification of Inderbitzen's method. Res. Vet. Sci., 29: 395-396. Powers, K., Wood, L., Eckert, J., Gibson, T. and Smith, H., 1982. World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) guidelines for evaluating the efficacy ofanthelmintics in ruminants (bovine and ovine). Vet. Parasitol., 10: 205-264. Roberts, F. and O'Sullivan, P., 1949. Methods for egg counts and larvae cultures for strongyles infesting gastrointestinal tract of cattle. Aust. J. Agric. Res., 1: 99-102.